You are to develop a software system for a Concert Ticketing System.

A customer may reserve a specific concert for a specific date, if available. To confirm

the reservation the customer must provide their credit card details. These details need

to be authorised by the appropriate credit card system. Tickets can be upgraded at

any time right up until the day of the concert, the customer must pay the price

difference via credit card. Due to COVID restrictions the customer must produce a

vaccination certificate when attending the concert. Door staff will scan the certificate

to ensure it is valid and only those with a valid certificate are permitted attend the

concert. A customer can get a refund if they test positive for COVID up to two weeks

prior to the concert date, a company employee ensures the supplied documentation

verifies the dates and issues the refund. After the concert the customer may leave a

review, if they wish, however they must have attended the concert to leave a review.